Broadband circularly polarized fanshaped beam antenna



Nov. 24, 1970 J. J. EPIS 3,543,276

BROADBAND CIRCULARLY POLARIZED FAN-SHAPED BEAM ANTENNA Filed April 10. 1-969 4 Sheets-Sheet 1 INVENTOR JAM ES J. EPIS B Mxm AGENT J. J. EPIS 3,543,276

4 Sheets-Sheet 2 Nov. 24, 1970 BROADBAND CIRCULARLY POLARIZED FAN-SHAPED BEAM ANTENNA Filed April 10, 1969 a W mm A N. W w NS G E M A III M NL M Y B \N \mm \\\\\\1% E: n X

Nov. 24, 1970 J. J. EPIS 3,543,276

BROADBAND CIRCULARLY POLARIZED FAN-SHAPED BEAM ANTENNA Filed April 10, 1969 4 Sheets-Sheet 3 INVENTOR.

E P 1 s I JAMES J.

AGENT Nov. 24, 1970 J. J. EPIS 3,543,276

BROADBAND CIRCULARLY POLARIZED FAN-SHAPED BEAM ANTENNA Filed April 10, 1969 4 Sheets-Sheet 4 VERTICAL POLARIZATION 6 odbs l0 s 20 HORIZONTAL POLARIZATION IE- El VERTICAL POLARIZATION AGENT United States Patent r 3,543,276 BROADBAND CIRCULARLY POLARIZED FAN- SHAPED BEAM ANTENNA James J. Epis, Sunnyvale, Calif., assignor to Sylvania Electric Products Inc., a corporation of Delaware Filed Apr. 10, 1969, Ser. No. 814,910 Int. Cl. H01q 19/00, 3/00 US. Cl. 343-756 7 Claims ABSTRACT OF THE DISCLOSURE This antenna comprises a circularly polarized conical horn that illuminates a cylindrical parabolic reflector. The horn has a relatively small aperture in one end thereof and a longitudinal axis that is tilted with respect to the reflector in the plane containing the directrix and focal line of the reflector, A quarter-wave plate and a resistanceabsorption card are oriented at 45 with respect to each other in a circular waveguide connected to the other end of the horn. The aperture of the horn is located on the focal line of the reflector.

BACKGROUND OF THE INVENTION The invention herein described was made in the course of or under a contract or subcontract thereunder, with the United States Army.

This invention relates to antennas and more particularly to a fan-shaped beam antenna that is capable of receiving electromagnetic wave signals of any linear polarization, Antenns used in azimuth direction finding (DF) applications receive signals from a number of different target antennas. Since the source of a particular received signal is often unknown, however, the following parameters defining the received signal are unknown:

(1) polarization orientation; (2) signal frequency;

(3) elevation angle; and

(4) azimuth angle.

In order to receive signals having any polarization orientation and to accurately determine the direction of the associated target antenna with a minimum number of antennas and associated receiver circuits, it is desirable that the DF antenna be circularly polarized, operate over a broad band of frequencies, and produce a beam having an azimuth boresight direction that is substantially independent of the polarization orientation of the received signal. The width of the beam in the elevation plane should be broad in order to receive a maximum number of signals. The width of the beam in the azimuth plane should be narrow, however, in order to precisely determine the azimuth direction of a target antenna.

In operation, the DF antenna is rotated in azimuth while the operating frequency of receiver equipment is swept over a band of frequencies at a rate that is much greater than the rotation rate of the DF antenna. Since the sweep rate is related to time constants of the receiver that are fixed, it is also desirable that the width of the azimuth beam be substantially independent of the frequency of a received signal.

An object of this invention is the provision of an im- Patented Nov. 24, 1970 proved directional antenna having a fan-shaped beam and satisfying the aforementioned considerations.

Another object is the provision of an antenna providing a circularly polarized fan-shaped beam.

Another object is the provision of a circularly polarized fan-shaped beam antenna having substantially constant half-power beam widths that are narrow in one plane and broad in an orthogonal plane over a frequency band greater than 1.5 :1.

A further object is the provision of a fan-shaped beam antenna wherein the angular direction of the azimuth boresight axis of the antenna beam is substantially independent of both the frequency and polarization of a received signal.

SUMMARY OF THE INVENTION In accordance with this invention, a circularly polarized fan-shaped antenna beam is provided by illuminating a cylindrical parabolic reflector with a feed having a center line that is tilted with respect to the reflector in a plane containing the directrix and focal lines of the reflector. The aperture of the antenna feed has a circular cross-section. In a preferred embodiment of the invention, the reflector is illuminated by a conical horn.

DESCRIPTION OF DRAWINGS FIG. 1 is a front view of an antenna embodying this invention;

FIG. 2 is a side view of the antenna of FIG. 1;

FIG. 3 is a section of the antenna feed assembly taken along line 3-3 in FIG. 2;

FIG. 4 is a section of the antenna feed assembly taken along lines 44 of FIG. 1;

FIG. 5 is a section of the antenna feed assembly taken along line 5-5 of FIG. 1;

FIG, 6 is a top view of the quarter-wave plate in the antenna feed assembly taken along line 66 in FIG. 5;

FIG. 7 is a side view of FIG. 6; and

FIGS. 8 and 9 are typical azimuth and elevation radiation patterns, respectively, illustrating the operation of the antenna of FIG. 1.

The scale factor of the illustrations in FIGS. 4-7 is twice the scale factor in FIG. 3, for illustrative purposes.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to FIGS. 1 and 2, this antenna comprises a reflector 1 which is illuminated by an antenna feed assembly 2. The reflector is secured to a plate 3 which is bolted to the top of an enclosure 4. The feed assembly is secured in a split bracket 5 which is also bolted to plate 3. The enclosure is mounted on a pedestal 6 which rotates the enclosure and thus the antenna and the fanv shaped beam produced thereby.

Reflector 1 is a cylindrical parabolic section that is tilted, see FIG. 2, so that the vertex line V thereof makes an acute angle a with respect to the vertical. The longitudinal axis XX of the feed assembly is located in the plane formed by a directrix D of the reflector and the vertex line V, i.e., in the plane of the paper in FIG. 2. This plane through the feed assembly and containing axis XX is normal to the plane of the paper in FIG. 1. As illustrated in FIG. 2, the feed assembly is tilted with respect to the reflector so that the axis XX forms an acute angle [3 with respect to the directrix D. This tilted orientation of the feed assembly (i.e., tilt feeding) is different from off-set feeding of a parabolic cylinder by a linear array of feeds to obtain electronic scanning of the antenna beam. In an off-set fed parabolic cylinder antenna, the longitudinal axis of each feed makes an angle with respect to its directrix D which is in a plane that is normal to the plane defined by the directrix and vertex lines of the reflector rather than in this latter plane in accordance with this invention.

The antenna feed assembly 2 is positioned so that the focal line F of the reflector passes through the aperture of the feed which is spaced the slant height it above plate 3. Thus, in azimuth DF applications in which the antenna is receiving signals, the principal ray R and reflected rays R each makes an angle 90-B with respect to the vertex line V. Electromagnetic wave signals are coupled between the antenna feed assembly and electronic receiving equipment such as a down-converter located in enclosure 4 by waveguide 7.

Referring now to FIG. 3, the antenna feed assembly comprises a rectangular to circular waveguide transition section 10, and waveguide sections 11 and 12. Waveguide section 11 is a split housing construction comprising two similar parts 14a and 14b, see FIG. 4. Each of the parts 14 has a semi-circular groove 15 therein so that a circular waveguide is formed when the parts 14a and 14b are bolted together. Recesses 16 and 17 are formed in the edges of the parts 14 and the surfaces of the grooves.

A resistance card 18 is located in the opening in section 11. Resistance card 18 may, by way of example, be a metalized sheet of mica having a constant thickness of approximately 0.005 inch for R-band. It is shown much thicker than this, however, in FIG. 4 for illustrative purposes. The resistance card is symmetrical about the center line XX of the waveguide and has tapered sides which terminate on the center line. Shoulders 19 and 20 on the center section of the card fit into the recesses 16 and 17, respectively, to locate and secure the card in the waveguide.

Waveguide section 12 comprises a length of circular waveguide 24 having one end 25 which is flared to form a conical horn that illuminates the reflector. A cover 26 that is made of Rexolite is secured over the horn aperture by a metal cap 27 to keep foreign material out of the feed assembly. The cover has a constant thickness of approximately 0.010 inch and has flat surface normal to the longitudinal axis of the feed assembly so that the cover does not function as a lens.

As illustrated in FIGS. 3 and 5, a quarter-wave plate 28 is located in circular waveguide 24. The quarter-wave plate is symmetrical about the center line XX of the feed assembly and is oriented at with respect to the resistance card and the plane defined by the lines D and V. Plate 28 is milled from a block of Rexolite and comprises a plurality of steps on opposite sides thereof (see FIGS. 6 and 7). The quarter-wave plate is press fitted into waveguide 24. Alternatively, the plate may be secured in the waveguide by dielectric screws (not shown) which extend through the waveguide wall and contact the edges of the plate. Waveguide transition 10 and section 12 are rigidly secured to opposite ends of waveguide section 11 by screws.

The operation of this antenna will now be described while considering the antenna as if it were transmitting electromagnetic wave signals. Consider that a linearly polarized wave is propagating in the dominant mode in rectangular waveguide 7 and circular waveguide section 11. This wave is not affected by resistance card 18 since the wave is polarized normal to the plane of the resistance card. Stated differently, the electric field of the dominant mode is perpendicular to the plane of the resistance card and is therefore not absorbed by the latter. Quarter-wave plate 28, however, is oriented at a 45 angle with respect to the plane of the resistance card 18 and the electric field of the linearly polarized wave. Thus, one component of the electric field is perpendicular to and another equal amplitude component is parallel to the plane of the quarter-wave plate. In propagating through circular waveguide 24, the phase of the electric field component that is parallel to the plane of the quarter-wave plate is delayed relative to the phase of the other orthogonal electric field component. Thus, an electromagnetic wave signal is radiated by conical horn 25. For the orientation of the quarter-wave plate illustrated in FIGS. 3 and 5 the ray R is right-hand circularly polarized. If the quarter-wave plate is rotated 90 in circular waveguide 24, signals radiated by horn 25 are left-hand circularly polarized. The ray R of the antenna has a polarization screw sense opposite to that of ray R in each case.

In practice, a small portion of the circularly polarized wave incident on the horn is reflected back toward the quarter-wave plate. This circularly polarized reflected wave has a sense opposite to that of the incident wave so that the former wave is converted to a linearly polarized wave. The electric field of this linearly polarized reflected wave is parallel to the plane of the resistance card and is therefor absorbed by the latter. If the resistance card were not present in waveguide section 11, the linearly polarized reflected signal would be reflected by rectangular waveguide 7 back toward plate 28. This new reflected wave would be converted by the quarter-wave plate to a third circularly polarized wave having a sense opposite to that of the original circularly polarized incident wave. Transmission of both of these circularly polarized waves would cause the polarization axial ratio of the antenna to increase. Thus, by employing resistance card 18 to absorb a small portion of the available signal energy, small values of polarization axial ratios are preserved.

The radiation pattern associated with the conical horn 25 is approximately a symmetrical surface of revolution about the horn axis XX. The horn has an aperture, however, that is located on the focal line of the cylindrical parabolic reflector and an axis XX that is tilted in the plane of the vertex and focal lines. In this configuration, radiation from the horn is diffracted by the reflector to form a fan-shaped beam having a narrow azimuth beamwidth in a plane parallel to the ground, see FIG. 8, and a broad elevation beamwidth in the vertical plane containing the focal and vertex lines, see FIG. 9.

Blockage of radiation from the reflector by the feed assembly is employed to advantage in this invention to shape the antenna radiation pattern in the vertical plane and thereby reduce the amount of radiation illuminating the earth over which the antenna is mounted. Specifically, the feed assembly is oriented at the angle )3 in front of a portion of the reflector so that the feed scatters the radiation incident on it after reflection from the parabolic cylinder 1. Thus, radiation from the antenna in the vertical plane at negative angles with respect to the horizontal, i.e., the angle 0 in FIG. 9, is reduced and the radiation pattern shape in the elevation plane is caused to be slightly asymmetrical with respect to the direction of maximum radiation, which is approximately along the line 30 that corresponds to the direction of the ray R in FIG. 2.. The reflector is tilted to make the angle a with respect to the vertical in order to make the relative signal level at zero elevation, i.e., at the earths surface, approximately 3 db below the maximum signal level of the beam in the elevation plane.

By way of example, an antenna embodying this invention that was built and tested had the following dimensions and characteristics:

Reflector 1 Vertex line length: 10.62 inches. Width: 6.825 inches. Focal length: 1.751 inches. Vertex angle a: 13 degrees.

Horn 25 Aperture diameter: 0.610 inch.

Taper angle 1: degrees.

Slant height h: 4.944 inches.

Tilt angle ,9: 32 degrees.

Aperture-to-reflector spacing: 1.751 inches.

Waveguide Sections 11 and 12 Inner diameter: 0.457 inches.

Quarter-wave plate 28 Width: 0.457 inch.

Total length: 1.111 inch.

Number of steps: 6.

Step

Length: s =0.139 s =0.144 s =0.159 s =0.l84 inch.

Thickness: t =0.072 t =0.055 t =0.035 1 0.010

inch.

Resistance card 18 Center length 1 0.78 inch.

Taper length 1,: 1.22 inches.

Taper angle 7: 10.6 degrees.

Thickness: 0.005 inch.

Resistivity: 200 ohms/ square.

Frequency band: 16 to 26 gHz.

VSWR (max.): 1.24: 1.

Polarization axial ratio Maximum: 2.4 db.

Nominal: 1.2 db.

Azimuth half-power beamwidth Nominal: 6.65 degrees. Variation: :08 degrees.

Elevation half-power beamwidth Nominal: 50 degrees. Variation: :3 degrees.

This antenna operated over a frequency bandwidth of greater than 1.6:1 in K -band. The aperture diameter was approximately 1% times the diameter of the air-filled circular waveguide 24, i.e., 1.03 Wavelengths, at the center frequency (21 gHz). This relatively small conical horn was located on the focal line of the reflector. The azimuth half-power beamwidth varied less than 112% as compared to :24% variation for a typical antenna (assuming that the beamwidth of the latter varies linearly with wavelength) operating over the same frequency bandwidth. Similarly, the elevation half-power beamwidth varied less than 16% as compared to 124% for a typical antenna. The maximum side lobe levels on the azimuth and elevation patterns weer at least -19 db and db, respectively, relative to the maximum of the beam. The maximum side lobe level of the more important and relatively narrow azimuth radiation pattern, however, was at most frequencies more than 30 db below the beam maximum. The direction of the boresight axis of the azimuth radiation pattern was substantially constant as the polarization of an incident signal was varied through all possible values over the frequency band of 16 to 24.5 gHz. The maximum variation of the direction of the boresight axis over the frequency band of 24.5 to 26 gHz was :055 degree from the nominal direction as a function of incident polarization orientation. The maximum axial ratio of this antenna was less than 2.4 db, with values of 1.2 db being more common. The polarization axial ratio was measured over a 60 section on the elevation pattern.

Although it is not known exactly why the azimuth and elevation beamwidth patterns (e.g., see FIGS. 8 and 9, respectively) are so nearly constant when the antenna is operated over a broad band of frequencies greater than 1.5 1, several design considerations related to this feature were determined empirically. As stated previously, it is desirable that a DP antenna receive signals having any linear polarization. Although a horn with an aperture having either a square or circular cross section will propagate a circularly polarized wave, and thus any linear polarization, it was discovered that in order to accomplish the objects of this invention the aperture of the antenna feed must have a circular cross section. In the preferred embodiment of this invention the antenna feed is a conical horn. It was determined that the diameter of the feed aperture must also be small, preferably between 0.95 and 1.25 wavelengths at the center operating frequency of the antenna, for the orientation of the boresight axis of the antenna to be independent of frequency and polarization orientation of received signals over a broad band of frequencies. It is also desirable that the slant height h spacing of the aperture be approximately 46% of the vertex length of the reflector, that the focal length of the parabolic reflector be between 2.8 and 3.2 wavelengths, and that the tilt angle B of the feed born be between 29 and 35.

When the reflector was illuminated by a feed horn with an aperture having a square cross section the azimuth boresight direction of the DF antenna varied appreciably (i.e., scanned as a function of the polarization of received signals) and the azimuth pattern beam shape was asymmetrical with respect to the scanned boresight direction. Also, the magnitudes of the side lobes and the polarization axial ratios were substantially greater than those obtained when the reflector was illuminated by a conical horn having an aperture of the correct size.

The superior performance of this antenna with a conical horn feed is believed to be achieved because cross polarized currents are induced on the reflector surface by two different phenomena and the resulting pair of cross polarized currents substantially cancel each other; whereas only one of these cross polarized currents is induced on the reflector when a square horn is used so that a net cancellation of highly undesirable effects due to cross polarized currents cannot be obtained. The second set of cross polarization currents produced on the reflector by a conical horn is obtained because the electric field lines therein curve so that they are normal to the periphery of the horn. In the square horn the corresponding field lines are straight.

What is claimed is:

1. A broadband circularly polarized fan-shaped beam antenna comprising a cylindrical parabolic reflector having vertex and focal lines that are parallel to each other and define a plane, said reflector having a directrix that is orthoginal to said lines in said plane,

a feed assembly having a longitudinal axis and oriented to illuminate said reflector, said feed assembly comprising a waveguide symmetrical with respect to said axis and capable of supporting a circularly polarized wave,

a resistance card in said waveguide, said card being symmetrical with respect to said axis,

a quarter-wave plate in said waveguide between said card and one end of said waveguide and oriented at a 45 angle with respect to said card,

first means having an aperture in one end thereof, said aperture having a circular cross section, the other end of said first means being connected to said one end of said waveguide,

second means supporting said feed assembly and said reflector in a fixed relationship,

said axis being in said plane and tilted with respect to said reflector to form an acute angle with respect to said directrix,

said one end of said first means being located proximate said reflector, and

third means for moving said reflector and feed assembly for moving the fan-shaped antenna beam produced thereby.

2. The antenna according to claim 1 wherein the diameter of the aperture in the one end of said first means is in the order of one wavelength at the center operating frequency of the antenna.

3. The antenna according to claim 1 wherein said first meansv is a conical horn.

4. The antenna according to claim 3 wherein the tilt angle between said axis and the directrix is between 2 9 and 35.

5. The antenna according to claim 4 wherein the focal line of said reflector passes through the aperture of said horn, the slant height of said aperture being approximately forty-six percent of the vertex length of said reflector as measured from one end thereof, said axis being tilted toward said one end of said reflector.

"6. The antenna according to claim 3 wherein said waveguide has a circular cross section.

7. The antenna according to claim 1 wherein the focal length of said reflector is between 2.8 and 3.2 wavelengths and the aperture diameter of said horn is between 0.95

8 of the operating frequency bandwidth of said broadband circularly polarized fan-shaped beam antenna.

References Cited UNITED STATES PATENTS 2,786,198 3/1957 Weil et al. 343756 X 3,233,241 2/1966 Alford 343-756 3,495,261 2/1970 Lastinger et al. 343766 X 10 HERMAN K. SAALBACH, Primary Examiner T. VEZJEAU, Assistant Examiner US. Cl. X.R.

Wavelength and 1.25 wavelengths at the center frequency 15 33321; 343-766, 7 83 786 840 

